1. Field of the Invention
The present invention relates to a fire discriminating apparatus so designed that the breaking out of a fire in an area under surveillance is discriminated on the basis of the temperature difference between a temperature detection value of the area and a reference temperature.
2. Description of the Prior Art
Apparatus heretofore known in the art to determine the braking out of a fire in accordance with the temperature difference between a reference temperature and a detected temperature include a so-called differential-type heat sensing apparatus having a diaphragm arranged in an air chamber.
In other words, the differential-type heat sensing apparatus is designed so that the air chamber is divided into a reference chamber sealed by the diaphragm and a detecting chamber communicated with the outside air through an orifice whereby when exposed to the heat due to a fire, the air in the detecting chamber is caused by thermal expansion to leak to the outside through the orifice and switch contacts are closed by a deformation of the diaphragm due to the pressure difference produced between the detecting chamber and the reference chamber. Then, this differential-type heat sensing apparatus is required to satisfy both of two functions, that is, on the one hand to give an alarm in less than 4.5 minutes against, for example, a temperature rise rate of 15 deg/min centigrade and on the other hand to give no alarm before the expiration of 15 minutes against, for example, a temperature rise rate of 3 deg/min centigrade.
On the other hand, in place of the above-mentioned mechanical type differential heat sensor, there has recently been proposed a heat sensing apparatus including a reference temperature sensor for reference temperature detecting purposes and another fire temperature sensor for detecting the actual temperature of an area under surveillance so as to detect a fire in accordance with the difference between the two detected temperatures. In other words, with this heat sensing apparatus, the reference temperature sensor is mounted within the sensing apparatus where it is not easily subjected to the effect of the temperature rise due to a fire, whereas the fire temperature sensor is mounted on a heat sensitive plate exposed to the outside of the sensing apparatus. As a result, when the ambient temperature rises due to a fire, the detected temperature of the reference temperature sensor rises slowly and the detected temperature of the fire temperature sensor rises in response to the actual temperature in the area under surveillance, thereby increasing the temperature difference between the two with the passage of time. This sensing apparatus generates a fire detection signal when the temperature difference exceeds a given threshold value.
With the differential-type heat sensing apparatus which determines the breaking out of a fire from the difference between the temperatures detected by the reference and fire temprature sensors, however, the variation characteristic of a reference temperature with respect to the variation of a fire temperature is determined by the heat conduction condition of the mouting mechanism of the reference temperature sensor so that the setting of this heat conduction condition must be effected by varying the mechanical conditions of the component parts and therefore it is difficult to accurately uniformly set a heat conduction condition for individual sensing apparatus. Thus, the fire detection characteristic varies among different sensing apparatus with the result that while the previously mentioned functions, i.e., the function of giving an alarm in less than 4.5 minutes against a temperature rise rate of 15 deg/min centigrade and the function of giving no alarm before the expiration of 15 minutes against a temperature rise rate of 3 deg/min centigrade are nearly satisfied, it is difficult to realize the desire that each of different sensing apparatus is accurately provided with the optimum fire discriminating characteristic which permits the early and accurate detection of a fire from the temperature difference between a fire temperature and a reference temperature.